1. Related Cased
This application represents an improvement on prior patent U.S. Pat. No. 5,018,593, issued May 28, 1991, the disclosure of which is hereby incorporated by reference.
2. Technical Field Of The Invention
The present invention relates to the technical field of heavy vehicles, such as ready mix concrete delivery trucks adapted for both roadway and off-road operation, and to the provision of lift axles for such vehicles to assist in spreading the vehicle weight, particularly when heavily laden, to minimize road damage and bridge damage.
The present invention relates in particular to the field of vehicles for carrying heavy loads, and particularly to improved lift axles for bearing and spreading a share of the lead in high vehicle gross weight conditions in order to comply with highway and bridge load-limitation laws and regulations, which retract to an inactive position when not required.
3. The Prior Art
Lift axles in a variety of configurations are coming into increasing use on heavy load-bearing trucks and related vehicles as means to comply with highway, road and bridge lead limits established by a diverse and complex array of laws and regulations. The dominant requirement for most purposes in the U.S. is the federal vehicle weight limits established by the federal Bridge Gross Weight Formula defined at 23 C.F.R. 658.5 and 658.17.
The federal Bridge Gross Weight Formula provides, in substance, that:
1. Maximum allowable vehicle gross weight is limited to 80,000 pounds. PA1 2. The maximum allowable lead on any single axle is limited to 20,000 pounds. PA1 3. The maximum allowable lead on any tandem axle assembly is limited to 4,000 pounds. PA1 4. The maximum allowable lead on any sequential series of axles must not exceed a weight W, defined by the formula: ##EQU1## W is the maximum allowable weight, in pounds; N is the total number of axles in the sequence;
L is the length of the wheel base between the first and last axles in the sequence.
The formula is made more complex by a series of definitions of single axles, tandem axles, and related provisions that govern the determination of L dimensions and other parameters.
The net effect of the federal Bridge Gross Weight Formula is to allow greater gross vehicle weights, up to the limit, both as overall wheel base is increased, and as the load is spread within that overall wheel base on several axles.
On the other hand, many jurisdictions have overall length restrictions on various types of vehicles. The common limit for a unit truck, i.e., a truck with a unitary non-articulating structure, is 40 feet overall length maximum in most states.
A complex maze of other requirements and limitations apply in various jurisdictions, and the result is a severe set of limitations on the payload of many types of vehicles.
Compliance is increasingly difficult to assure. Noncompliance brings high fines and penalties.
As one type of truck greatly limited by the regulations and laws, ready mix concrete delivery trucks are more often limited in payload by the federal bridge Gross Weight Formula than any other parameter. In most circumstances, a typical three axle ready mix concrete truck will be limited to a legal capacity of about six cubic yards of concrete mix. Equipped with five or even up to six axles and lightweight mixer drum assembly construction, the legal capacity has been extended to 8 to 81/4 cubic yards. More recently, we have been able to offer over ten cubic yards of ready mix concrete pay load in trucks fully compliant with the federal Bridge Gross Weight Formula and with an overall length that does not exceed the common forty foot limitations of many states. Such load spreading is not necessary or desirable in off-road operation, such as at a job site where a load of concrete is to be discharged, for example.
As a general rule, the greater the number of axles, the higher the allowable gross vehicle weight, and thus the net pay load of the vehicle. On the other hand, adding additional axles to a vehicle leads to other disadvantages. Higher rolling resistance, losses in fuel economy, engine wear, steering and handling difficulties under some conditions, losses in traction by the driven wheels in some conditions, and the like all serve to make added axles difficult and expensive.
It is possible to have the advantages of added axles, and to eliminate most of the serious disadvantages by the use of lift axles, designed to be deployed in a down position to bear a portion of the load in highway service, and to be retracted to a level out of contact with the road or ground for off-road operations or for highway operation in unloaded or light load conditions when the weight bearing capacity is not needed.
Lifting the tag axles or other forms of lift axles serves to decrease the load imposed on the front axle of the truck, making steering loads lighter, while increasing the weight and traction on the driving wheels, and shortening the wheel base to aid maneuverability. In addition, tire wear is reduced while rolling resistance and fuel consumption are decreased in off-road operations and in highway operating conditions in light load or unloaded conditions. We also recommend that the lift axle be retracted at any time the vehicle is backed, and prefer to provide for automatic retraction whenever reverse gear is selected by the operator of the vehicle.
There are two major types of lift axles in use. These are under-frame systems, typified by the units available from Suspensions, Inc., and Hi Steer Canada, Ltd., and so-called high lift models, offered by a number of truck builders and suppliers, of which our prior U.S. Pat. No. 5,018,893 is an example.
The under-frame systems are typically pneumatic suspension and retraction mechanisms. Such "air ride" suspensions have been highly developed in recent years and have excellent performance characteristics. As lift axles, these systems are ordinarily based on pivotally mounted leading or trailing links, depending on the location of the assembly. When the link leads the axle, these units are known as "tag" axles. When the link trails the axle, often employed when the assembly is mounted ahead of a tandem axle pair, the unit is normally called a "pusher" axle.
As illustrated in our prior patent, supra, the high-lift systems are in tag axle configuration, and are normally hydraulically actuated. The suspension system may be hydraulic or pneumatic.
The under-frame systems limit ground clearance, and have limited lift clearance, ordinarily no more than about 18 inches. It is often difficult to mount a rear under-frame tag axle in a manner such that the center is more than 96 inches (2.43 m) behind the center of the next forward axle. As a major factor in the L measurements under the federal Bridge Gross Weight Formula, the benefits of a tag axle may be limited in such a case.
The high-lift systems are mounted above the frame, retract to well above the frame, and have excellent ground clearance, but are more prone to failures. Problems generally arise because of the length of the link arms, and because the suspension is provided by the hydraulic system in most units of this type. In prior U.S. Pat. No. 5,018,593, a separate pneumatic suspension system resolves a portion of the problem, but because the suspension is mounted on the link arm and not on the frame of the vehicle, the long lever arm of the links transmits the entire load and is still a considerable problem. The weight of such units is undesirably but necessarily high.
When the vehicle passes across a depression of sufficient span and depth, it is sometimes possible for the entire weight of the vehicle to be placed on the foremost and the rearmost tag axle. With the vehicle in motion, the inertial loading of such a condition can be several times the static load represented by the gross vehicle weight, and failure of the tag axle or the truck frame can result.
Such conditions can occur with substantial frequency for construction vehicles and other combined highway and off-road vehicles. Typical of these is a ready-mix concrete delivery truck, for example, which operates from a ready-mix concrete plant to a job site over the highways, and while at the job site to deliver and discharge its load, is expected to traverse the site whatever its condition may be. There are frequent occasions to cross drainage ditches, swales and other depressions in the topography on the site. If, through operator haste and error, the lift axle is not retracted to its lifted position, front-rear axle bridging may occur. At times, if this occurs at substantial speeds, it results in dynamic inertial loading far in excess of the design requirements for such equipment. Gross vehicle weights of 60,000 to 70,000 pounds (27,272.73 to 31,818.18 kg) are common to such vehicles. Inertial loading in a bridged condition can, in some circumstances, reach as high as 30,000 pounds (13,636.36 kg) or even more, over a beam length spanning, from the front axle to the rear axle, of as much as 30 feet (9.23 m). Under such conditions, failure of the lift axle assembly or the truck frame is inevitable.
In prior application Ser. No. 07/957,030, filed Oct. 10, 1992, we provided an improved lift axle assembly which provides for safer operation and more effective usage of such lift axles, wherein the suspension system of the lift axle assembly is deactivated in overload conditions to prevent damage to the vehicle and to the lift axle assembly in bridging conditions. That invention may be, and preferably will be, employed in combination with the present invention.
In the prior art, and even in our prior invention in Ser. No. 07/957,030, there is an additional problem which is not resolved, that of providing adequate ground clearance when the lift axle of the "under frame" type is retracted to its lifted position.
Particularly when operating off the road, on a construction site or the like, the ground clearance afforded by most under frame lift axles is limited to about 18 inches (45.72 cm); in our prior application, we increased the clearance to about 22 to 24 inches (55.88 to 60.96 cm). While that increase was a welcome and significant improvement, usage indicates that it is still less than desirable and that impacts of the lifted axle and its wheel sets and tire with obstructions on the job site remains a problem.